Stable platelet adhesion, aggregation and thrombus formation are dependent on inside-out signals that activate and regulate adhesive ligand binding to integrin alphaIIb-beta3, and on outside-in signals from alphalI-beta3 that regulate cytoskeletal events in platelets. Defects in alphaIIb-beta3 signaling can cause a bleeding diathesis, but the molecular basis of these defects remains unclear, as does the role of alphaIIb-beta3 signaling in megakaryocytes, cells from which platelets are derived. The goal of this project is clarify the molecular basis of alphalIb-beta3 signaling in platelets and megakaryocytes by focusing on two major classes of protein kinases, Src and protein kinase C (PKC), which have been shown recently to interact with alphalIb-beta3. This work will address three specific alms by utilizing a unique experimental system, mouse megakaryocytes and platelets derived from embryonic stem cells or bone marrow, in conjunction with gene expression, knock-down and knock-out strategies. First, we will test the hypothesis that c-Src is regulated and outside-in signaling is initiated in megakaryocytes and platelets by direct interaction of c-Src with the beta3 cytoplasmic tail. These experiments will evaluate the structural basis of c-Src regulation by alphallb-beta3, the role of tyrosine phosphatases in this interplay, and the idea that Src activation can be disrupted selectively without impairing other alphaIIb-beta3 signaling functions. Second, the mechanisms and consequences of PKC interaction with alphalIb-beta3 will be investigated. In particular, the interaction of specific PKC isoforms with alphalIb-beta3 will be characterized, and their functions in inside-out or outside-in alphaIlb-beta3 signaling established. The possibility that one or more PKCs regulate talin, another integrin-binding protein implicated in alphalIb-beta3 activation, will be assessed. Third, possible roles for alphaIl-beta3, Src and PKC in megakaryocyte maturation and proplatelet formation will be investigated. Thus, by establishing the mechanisms and consequences of alphaIIb-beta3/Src/PKC interactions in the relevant primary murine cells, these studies should provide new insights into normal megakaryocyte and platelet function, into platelet dysfunction during defective hemostasis, and into identification of new therapeutic targets for pharmacological blockade of alphalIb-beta3 signaling in arterial thrombosis.